This application is a U.S. NATIONAL PHASE APPLICATION OF PCT INTERNATIONAL APPLICATION PCT/JP00/04420.
The present invention relates to a secondary battery using an aqueous electrolytic solution containing a lithium salt dissolved therein.
Recently, lithium batteries are widely used for main powers of mobile telecommunication gears and portable digital assistants, as batteries with high electromotive force and energy density. In general, these batteries are structured so as to use such compounds as lithium-containing oxides capable of intercalating and de-intercalating lithium ions and exhibiting high electrode potential, e.g. LixCoO2 and LixMnO2, as their positive electrode active materials, and to use lithium metal or other materials such as graphite and amorphous carbon capable of intercalating and de-intercalating lithium ions and exhibiting low electrode potential like as lithium, as their negative electrode active materials. When these active materials are used in aqueous electrolytic solution, the reaction of lithium and water makes low potential as essential potential of lithium unstable and hinders the active materials from intercalating or de-intercalating lithium ions, and accompanies water electrolysis that makes the voltages lower than that at which the water electrolysis occurs. For these reasons, non-aqueous electrolytes that contain no water in a strict sense are generally used for electrolytic layers.
Various forms are known as these non-aqueous electrolytes. Such forms include: organic electrolytes containing lithium salts dissolved in organic solvents; lithium-ion conductive solid electrolytes; so called xe2x80x9cgelxe2x80x9d polyelectrolytes containing organic electrolytic solutions held in polymeric matrixes; and dry electrolytes containing lithium dissolved in such organic polymers as polyethylene oxides. As described above, the use of non-aqueous electrolytes inhibits the reaction of lithium and water and makes the electrode reaction caused by lithium intercalation and de-intercalation stable. The use of non-aqueous electrolytes also avoids water electrolysis, and thus a high cell voltage far exceeding 1.2 V of decomposition voltage of water and almost reaching 3 to 4 V can be obtained in a stable manner. On the other hand, however, batteries using these non-aqueous electrolytes have problems related to their properties and safety peculiar to the form of each electrolyte.
Among the various non-aqueous electrolytes shown above, organic electrolytes are most highly used for lithium secondary batteries. Typical organic electrolytes widely used are those containing such lithium salts as lithium hexafluorophosphate and lithium tetrafluoroborate dissolved in such organic solvents as ethylene carbonate, diethyl carbonate, and dimethyl carbonate. In general, these organic solvents are highly voltaic and combustible. For such a reason, batteries using such organic electrolytes have risks that they might be exploded by an increase in their internal pressure or may ignite by catching fire if an abnormal increase in the temperature of the batteries or short-circuit inside of the batteries should be caused by some reasons. In addition, there is another problem. In storage or transportation of combustible or flammable organic solvents and electrolytes and batteries using such solvents, many limitations are imposed on their amount and handling conditions; therefore, their productivity and forms of transportation are restricted.
The above safety problems of combustible organic solvents can be avoided by the use of inorganic solid electrolytes instead that are not voltaic or flammable in themselves, such as Li3PO4xe2x80x94Li2Sxe2x80x94SiS2 and Li3N. In order for these inorganic solid electrolytes to serve as good electrolytes in the form of batteries, the solid electrolytes must be milled, mixed with electrode materials, and molded. Therefore, they have such problems as contactability to active materials and moldability liquid electrolytes never have. The problems arise only because they are solid.
On the other hand, xe2x80x9cgelxe2x80x9d electrolytes that hold organic electrolytic solutions mixed with such a polar polymeric component as polyacrylonitrile in polymeric matrixes are disclosed, for example, in Japanese Patent Non-Examined Publication Nos. H04-306560 and H07-82450. Such a xe2x80x9cgelxe2x80x9d electrolyte has immobilized electrolytic solution, which has improved handling apparently. However, since this kind of xe2x80x9cgelxe2x80x9d polyelectrolytes are the same as organic electrolytic solutions in using organic solvents, problems concerning safety, storage, and transportation essentially have not been solved. In addition, since the polymeric components increase ionic transfer resistance, batteries using xe2x80x9cgelxe2x80x9d polyelectrolytes tend to be worse than those using organic electrolytic solutions in performance.
As other electrolytes, xe2x80x9cdryxe2x80x9d polyelectrolytes were developed. They are electrolytes containing lithium salts dissolved in, for example, polyethylene oxide, and an electrolyte as disclosed in Japanese Patent Non-Examined Publication No. H10-204172. This electrolyte has a structure of a cross-linking polymer of a polyether copolymer together with a solute dissolved therein. These xe2x80x9cdryxe2x80x9d polyelectrolytes, however, have an electric charge transfer mechanism in which mobile cations, i.e. lithium ions, and anions, i.e. counter ions of the lithium ions, transfer at the same time. This mechanism lowers the cation transferenece number, thus posing a problem that transfer of the substances are rate-determining and rapid charge/discharge characteristics are unsatisfactory.
Among these various problems of non-aqueous electrolytes, those concerning safety that may lead to such accidents as explosion, catching fire, and ignition should be addressed first from a practical point of view. In order to avoid these safety problems and improve ionic conductivity as well, the application of aqueous electrolytic solutions that have no risks of catching fire or ignition in them and have excellent ionic conductivity is ideal. Then, some concepts of lithium secondary batteries using conventional aqueous electrolytic solutions were disclosed.
For instance, in Japanese Patent Application Non-Examined Publication No. H09-508490, a lithium secondary battery was disclosed, using a compound that intercalates and de-intercalates lithium ions and exhibits an electrode potential higher than that of lithium, such as LiMn2O4, or VO2 for its positive electrode active materials, and a compound that intercalates and de-intercalates lithium ions and exhibits an electrode potential similar to that of lithium, for its negative electrode active materials, and an alkali aqueous electrolytic solution containing a lithium salt such as LiCl and LiOH dissolved in water.
Moreover, released in a newspaper on Jun. 9, 1999, was a lithium secondary battery using, LiCoO2, LiNiO2, LiMn2O4, or LiV2O5 for its positive electrode active materials, and a vanadium compound such as LiVO2, LiV3O8, or iron compound such as xcex3-FeOOH for its negative electrode active materials, and a neutral aqueous electrolytic solution containing lithium sulfate or lithium chloride dissolved in water as its electrolytic solution.
However, according to the disclosure, each of these types of batteries was only structured to use an aqueous electrolytic solution in place of a non-aqueous electrolyte in the conventional lithium secondary batteries using non-aqueous electrolytes, and had operating voltages from 1 to 2 V. In reality, the stable operating voltage range was approximately 1.2 V and these aqueous electrolyte batteries essentially did not exceed the concept of those conventional batteries using aqueous electrolytes. Therefore, it had been considered that the batteries using aqueous electrolytic solutions were not capable of attaining such a high electromotive force attainable with lithium batteries using non-aqueous electrolytes.
The present invention intends to realize a lithium secondary battery that has an aqueous electrolytic solution but exhibits high electromotive force attainable with a non-aqueous electrolyte secondary battery by paying attention to the following two points. The high cell voltage of a lithium secondary battery is obtained by an environment of a material intercalating and de-intercalating lithium ions and an ion-conductive electrolyte without existence of water; and water electrolysis in the aqueous electrolytic solution is caused by the transfer of electrons between the electron-conductive electrodes and water molecules in contact therewith. Moreover, the present invention also intends to realize a lithium secondary battery having a high level of safety in which no internal short-circuit is caused by dendrite growth, utilizing a feature of no electrical deposit of metal lithium in aqueous electrolytic solutions.
In the present invention, disclosed is a lithium secondary battery having, as its basic structure to achieve the above purposes, positive and negative electrodes having active substances capable of intercalating and de-intercalating lithium ions; electrode coating layers made of a water insoluble ion-conductive polymeric solid electrolyte covering the both electrodes; and an aqueous electrolytic solution that exists between the electrode coating layers on the above both electrodes and is separated from the positive and negative electrodes.
In the structure of the present invention, like non-aqueous electrolyte batteries, reversible electric potential is generated by intercalation and de-intercalation of lithium ions from the positive and negative electrodes coated with the non-aqueous polymeric solid electrolyte. The aqueous electrolytic solution exists between the polymeric solid electrolytic layers covering each of the positive and negative electrodes, and is only assigned for ionic conductivity of lithium ions. Because each of the above positive and negative electrodes is separated by the non-aqueous electrolyte lithium from the aqueous electrolytic solution, no electrons are transferred between water molecules and the electrode plates. Therefore, no electrolysis will occur even if an electric potential higher than that of water electrolysis is generated between the positive and negative electrodes. This structure has enabled the accomplishment of a lithium secondary battery having safety and a cell voltage exceeding 3 V that had been considered attainable only with conventional non-aqueous electrolytes even though the battery uses an aqueous electrolytic solution.